1. Field of the Invention
The present invention relates to a sheet-medium conveying apparatus including a conveying belt that conveys a sheet-medium by electrostatic adsorption.
2. Description of the Related Art
An ink-jet recording apparatus that performs printing by depositing droplets of a recording liquid onto a sheet recording medium while conveying the sheet recording medium, by using a recording head, for example, which includes a liquid discharging head that discharges a droplet of a recording liquid, is known as an image forming apparatus, such as a printer, a facsimile apparatus, a photocopier, and a multi-function peripheral.
To achieve a high image quality, the precision of a spotting position of an ink droplet on a recording medium needs to be improved, and a structure of a recording head that jets an ink droplet and a recording medium need to be conveyed with high precision.
To improve the conveyance precision of a recording medium, proposed is an ink-jet recording apparatus of a so-called charged-belt type that is configured to convey a recording medium by adsorbing the recording medium electrostatically with a charged endless conveying belt, and revolving the conveying belt in the state that the recording medium is adsorbed.
First of all, an ink-jet recording apparatus of a conventional charged-belt type is explained below.
FIGS. 14 and 15 are a plan view and a side view, respectively, of a configuration of and around a conveying belt according to the ink-jet recording apparatus of the conventional charged-belt type.
The amount of revolution of a driving roller 12 that drives a conveying belt 11 is detected by an encoder 13 that is provided at an end of the driving roller 12; and then a control unit 14 drives a sub-scan motor 16 by controlling a driving unit 15 in accordance with the detected amount of revolution, and controls output of an alternating current (AC) power unit (AC bias supply unit) 18 for applying a high voltage (AC bias) onto a charging roller 17.
At that time, as the AC power unit 18 controls a cycle (application time) of an AC voltage to be applied onto the charging roller 17, and the control unit 14 controls driving of the conveying belt 11 at the same time, positive and negative charges can be applied onto the conveying belt 11 with a certain charge-cycle length. The “charge-cycle length” means a width (distance) in a conveying direction per cycle of an AC voltage to be supplied.
When starting printing, the conveying belt 11 is revolved clockwise in FIG. 15 by rotationally driving the driving roller 12 with the sub-scan motor 16, and at the same time, a square wave of which polarity alternates between positive and negative is applied onto the charging roller 17 from the AC power unit 18. Because the charging roller 17 is in contact with an insulating layer of the conveying belt 11, a positive charge and a negative charge are applied to the insulating layer of the conveying belt 11 alternately in a directional orthogonal to the conveying direction (sub-scan direction) of the conveying belt 11. As shown in FIG. 14, as a result, a positively charged area 101 and a negatively charged area 102 are formed alternately in belt-like stripe in regular width.
When the insulating layer of the conveying belt 11 has a volume resistivity of 1012 Ωcm or higher, preferably a volume resistivity of 1015 Ωcm, it is possible to prevent a positive charge and a negative charge from moving beyond a boundary, and keep positive and negative charges applied on the insulating layer.
Principles of charging are explained below with reference to FIG. 16.
When a charged dielectric is placed in an electric field, a Coulomb force (F=qE) is generated in a charge in the dielectric. The force is exerted on both a true charge and a polarized charge, so that a force received by the dielectric from the electric field is expressed with Maxwell stress tensor (the following Equations (1) to (4)).
                    F        =                              ∫                                          (                                  ρ                  +                                      ρ                    p                                                  )                            ⁢              E              ⁢                              ⅆ                v                                              =                                    ∫                                                E                  ·                                      (                                                                  ɛ                        ·                        div                                            ⁢                                                                                          ⁢                      E                                        )                                                  ⁢                                  ⅆ                  v                                                      =                          ∫                                                T                  ·                  n                                ⁢                                  ⅆ                  s                                                                                        (        1        )                                T        =                              ɛ            0                    ⁡                      (                                                                                                      E                      x                      2                                        -                                                                  1                        2                                            ⁢                                              E                        2                                                                                                                                                        E                      x                                        ⁢                                          E                      y                                                                                                                                  E                      x                                        ⁢                                          E                      z                                                                                                                                                              E                      y                                        ⁢                                          E                      x                                                                                                                                  E                      y                      2                                        -                                                                  1                        2                                            ⁢                                              E                        2                                                                                                                                                        E                      y                                        ⁢                                          E                      z                                                                                                                                                              E                      z                                        ⁢                                          E                      x                                                                                                                                  E                      z                                        ⁢                                          E                      y                                                                                                                                  E                      z                      2                                        -                                                                  1                        2                                            ⁢                                              E                        2                                                                                                                  )                                              (        2        )                                n        =                  (                                                                      n                  x                                                                                                      n                  y                                                                                                      n                  z                                                              )                                    (        3        )                                F        =                  (                                                                      F                  x                                                                                                      F                  y                                                                                                      F                  z                                                              )                                    (        4        )            
In the above equations, ρ denotes a true charge per unit of volume (volume density of charge c/m3), ρp denotes a polarized charge per unit of volume (volume density of charge c/m3), ∈ denotes the permittivity of the dielectric, and ∈0 denotes the permittivity in a vacuum.
A force acting on printing paper 19 shown a schematic diagram in FIG. 16 is obtained in two dimensions by using the above equations. Where it is assumed that a region of the printing paper 19 is denoted by S, an adsorption force acting on the paper is denoted by Fy, and influence in the thickness direction of the printing paper 19 is ignored; the surface vector of a boundary surface S1 between the printing paper 19 and the conveying belt 11 (vertical to the surface and outward) can be expressed by n1=(0, −1.0). Accordingly, the following Equation (5) is obtained.
                                                        Fy              =                              ∫                                                      {                                                                  ɛ                        0                                            ·                                              (                                                                              Ey                            ·                            Ex                                                    ,                                                                                    Ey                              2                                                        -                                                                                          1                                2                                                            ⁢                                                              E                                2                                                                                                              ,                                                      Ey                            ·                            Ez                                                                          )                                            ·                                              (                                                  0                          ,                                                      -                            1                                                    ,                          0                                                )                                                              }                                    ⁢                                      ⅆ                    s                                                                                                                          =                              ∫                                                                            ɛ                      0                                        ·                                          (                                                                        -                                                      Ey                            2                                                                          +                                                                              1                            2                                                    ⁢                                                      E                            2                                                                                              )                                                        ⁢                                      ⅆ                    s                                                                                                          (        5        )            
Because the above equation is two-dimensional, when Ez=0, the following Equation (6) is obtained:E2=Ex2+Ey2+Ez2=Ex2+Ey2  (6)
The following Equation (7) is obtained by substituting Equation (6) into Equation (5):
                              Fy                      (                          S              ⁢                                                          ⁢              1                        )                          =                  ∫                                                    ɛ                0                            ·                              (                                                                            1                      2                                        ⁢                                          Ex                      2                                                        -                                                            1                      2                                        ⁢                                          Ey                      2                                                                      )                                      ⁢                          ⅆ              s                                                          (        7        )            
From Equation (7) it is clear that, when Ex<Ey, the adsorption force is large.
Similarly, the surface vector of a printing surface S2 is (0, 1.0), accordingly, the following Equation (8) is obtained:
                                                        Fy              =                              ∫                                                      {                                                                  ɛ                        0                                            ·                                              (                                                                              Ey                            ·                            Ex                                                    ,                                                                                    Ey                              2                                                        -                                                                                          1                                2                                                            ⁢                                                              E                                2                                                                                                              ,                                                      Ey                            ·                            Ez                                                                          )                                            ·                                              (                                                  0                          ,                          1                          ,                          0                                                )                                                              }                                    ⁢                                      ⅆ                    s                                                                                                                          =                              ∫                                                                            ɛ                      0                                        ·                                          (                                                                        Ey                          2                                                -                                                                              1                            2                                                    ⁢                                                      E                            2                                                                                              )                                                        ⁢                                      ⅆ                    s                                                                                                          (        8        )            
Accordingly, when Ez=0, the following Equation (9) is obtained:
                              Fy                      (                          S              ⁢                                                          ⁢              2                        )                          =                  ∫                                                    ɛ                0                            ·                              (                                                                            -                                              1                        2                                                              ⁢                                          Ex                      2                                                        +                                                            1                      2                                        ⁢                                          Ey                      2                                                                      )                                      ⁢                          ⅆ              s                                                          (        9        )            
From Equation (9) it is clear that, when Ex<Ey, the adsorption force is large.
From the above discussion, to obtain an adsorption force, it is found that an electric field vertical to the paper is required on the boundary surface S1 between the conveying belt 11 and the printing paper 19, and an electric field horizontal to the paper is required on the printing surface S2. In other words, even if forming an electric field that is uniform in the vertical direction of paper, an adsorption force is not generated, so that an adsorption force needs to be maintained by forming an electric field as described above while forming a non-uniform electric field.
When applying positive and negative charges alternately on the conveying belt 11, as shown in FIG. 17, lines of electric force are not generated to be uniform only in one direction, so that the electric field is not to be uniform. This corresponds to the non-uniform electric field described above.
When the printing paper 19 is charged under a state where an electric field as described above is formed in this way, internal polarization arises in the printing paper 19. Moreover, because the printing paper 19 is not an insulator, a true charge also moves in accordance with the electric field. However, the electric field is not uniform, a charge moves biasedly along the lines of electric force. For example, with respect to the negatively charged area 102 shown in the center of the figure in which the conveying belt 11 is negatively charged, a positive charge appears on the boundary surface between the printing paper 19 and the conveying belt 11. However, on the surface side (printing surface) of the printing paper 19, the electric field is dense in the vicinity of a boundary surface between a positive polarity and a negative polarity of the conveying belt 11, on the contrary, it is sparse in the center, so that a charge appears only in a part where the electric field is dense.
In this way, differences are brought about because conditions of generation of a charge vary between the front side and the back side of the printing paper 19, consequently, a downward adsorption force is generated as explained above with reference to FIG. 16.
To make it clearly understandable, the strengths of the electric field are indicated by arrows in FIG. 18. As is clear from the figure, on the boundary surface S1 between the conveying belt 11 and the printing paper 19, the strength of the electric field in the vertical direction is high in the vicinity of each boundary surface between a positive polarity and a negative polarity of the conveying belt 11. For this reason, on the contrary, the strength is low in the center part where each charge area is stable. Also on the front side (printing surface) S2 of the printing paper 19, the strength of the electric field in the horizontal direction is the highest in the vicinity of each boundary surface between a positive polarity and a negative polarity of the conveying belt 11. Similarly, the strength is low in the center part of the conveying belt 11. In other words, because the electric field required for adsorption is the greatest in the vicinity of each boundary part between a positive polarity and a negative polarity of the conveying belt 11, it is clear that an adsorption force in the part is the strongest.
Returning to explanation of FIGS. 14 and 15.
The printing paper 19 is sent to the conveying belt 11 on which a non-uniform electric field is generated by forming positive and negative charges on the conveying belt 11. The printing paper 19 is conveyed in a state that the printing paper 19 is pressed onto the conveying belt 11 by pressure rollers 21.
Inside the printing paper 19 sent onto the non-uniform electric field on the conveying belt 11, movement of a charge occurs along a direction of the electric field. Precisely, the electric field toward a recording head (not shown) provided under ink cartridges 20 is reduced. Moreover, a charge applied on the surface of the conveying belt 11 and a charge that has an electric repulsion against the charge on the conveying belt 11 are reduced on the surface of the printing paper 19, consequently, an adsorption force of the printing paper 19 to the conveying belt 11 increases with time.
FIG. 19 is a graph of a temporal transition of adsorbability with respect to humidity as a parameter until a certain adsorption force is generated. According to the graph, it is clear that an adsorption force is a function of time. In other words, the graph indicates that it takes a long time to move a charge inside the printing paper 19. Moreover, it is found that movement of a charge takes a long time when the value of resistance of the printing paper 19 is high, because the value of resistance of paper is generally low in high humidity, by contrast, it is generally high in low humidity. For this reason, when the value of resistance is high, and a conveying speed is high, an adsorption force is insufficient, so that a strong electric field needs to be formed by increasing a voltage, as a result, charge irregularity tends to occur (the reason for this will be described later with reference to FIG. 20).
As described above, the printing paper 19 adsorbed onto the conveying belt 11 is conveyed to a position below the recording head, and then an image corresponding to one reciprocation of the head is formed on the printing paper 19 as the ink cartridges 20 is reciprocated in the main-scan direction, and at the same time ink droplets are discharged by the recording head. When an image corresponding to one reciprocation is formed, the printing paper 19 is sent to the next printing position by the conveying belt 11, and then image formation corresponding to one reciprocation is carried out.
When a leading end of the printing paper 19 reaches the position of driven spurs 22, the printing paper 19 is held by the pressure rollers 21, the conveying belt 11, and the driven spurs 22. When printing a trailing end of the printing paper 19, the printing paper 19 goes out from the pressure rollers 21 first, and the printing paper 19 is held by the driven spurs 22 and the conveying belt 11. When printing operations equivalent to one sheet are finished, the printing paper 19 is conveyed as it is by the conveying belt 11, and delivered.
However, as shown in FIG. 18, because the adsorption force becomes the largest at the boundary between a positive polarity and a negative polarity, and the smallest at the center of each of a positive polarity and a negative polarity, the adsorption force at the leading end of paper and that at the trailing end of the paper are lower except when the boundary between a positive polarity and a negative polarity is at the leading end or the trailing end. Although a holding force can be maintained by enhancing a charge (specifically, an applied charge is increased), the electric field becomes strong, resulting in influence on discharge of ink in a printing area in some cases.
Such situation is explained below with reference to FIGS. 20A and 20B. FIG. 20A depicts a condition of spotting when discharging ink at regular intervals without influence of an electric field, and FIG. 20B depicts a condition of spotting when discharging ink at regular intervals under a state where an electric field is formed.
Although it is empirically found that a discharged ink droplet (particularly, in a case of a small droplet, which is easily influenced) tends to have a negative charge); if there is no influence of any electric field, ink droplets 104 that is discharged spots at a desired position on the printing paper 19 as shown in FIG. 20A. However, if an electric field is formed, as shown in FIG. 20B, the ink droplets 104 that have a negative charge are influenced by the electric field on printing paper, and spotting positions are deviated, thereby causing image abnormality called charge irregularity. The charge irregularity is remarkable when the value of resistance of the printing paper 19 is high and the ink droplets 104 are small under a particularly strong electric field.
As a measure other than increasing the applied voltage, a method of controlling so as to align the leading end and the trailing end of the printing paper 19 with a charge boundary surface is conceivable; however, such method is restricted by the length of the printing paper 19 and the charge-cycle length, therefore, when one of the leading end and the trailing end is aligned with the charge boundary surface, the other may miss timing.
The problems described above can be summarized in the following paragraphs (1) and (2).
(1) In the conventional charged-belt method, the leading end and the trailing end of printing paper in the conveying direction have one of a positive polarity and a negative polarity. Because an adsorption force acts most strongly at the boundary between a positive polarity and a negative polarity, there is a problem that the adsorption force is consequently weaker at the leading end and the trailing end.
(2) Because the amount of charge varies depending on electric characteristics of the printing paper and environmental variations, an adsorption force changes. Although application of a strong electric field can be conceivable in order to accept change in the adsorption force, the electric field influences characteristics of discharging ink, thereby causing an abnormal image in stripe, which is called a nonuniform-charge image, or staining the printing head with mist generated when discharging ink, and consequently causing an abnormal image, such as a discharge fault or a frictional image.